A conventional EPROM-type flash memory features a byte-programming and a block-erasing capability with each block containing a number of bytes. Because the data within a memory block can not be selected for erasure individually, a flash memory array has to erase the data of a whole block of memory cells, i.e., an erased block, and then program the new data byte by byte. The block erasing scheme, however, not only is inflexible but also has an undesirable problem called over-erasure that results from simultaneously erasing memory cells requiring different erasing time.
As is well known, it is very important for a flash memory device to provide efficient erasing and repairing operations without shortening the life of the device. Towards achieving this goal, different bias conditions to the cells in the memory array as well as improved decoder circuits for selecting word lines and providing appropriate bias conditions to the cells during erasing and repairing operations have been researched and pursued.
As shown in FIG. 1(a), a widely used erasing technique generally referred to as channel erasing applies a positive high voltage to the bulk (substrate) 101 of a memory cell and grounds its control gate 102. This bias condition builds a high vertical electrical field to induce electron tunneling effect that causes the injection of electrons toward the bulk from the floating gate 103 of the device and reduces its threshold voltage.
The bias condition is reversed in a repairing operation. As shown in FIG. 1(b), a positive high voltage is applied to the control gate 102 and the bulk 101 is grounded to inject electrons back to the floating gate 103. The channel erasing or repairing technique requires a relatively high voltage to be applied to the bulk 101 or the control gate 102 of the device in order to achieve a fast erasing or repairing speed.
The decoder circuitry of a memory array can be divided as an X-decoder circuit and a Y-decoder circuit. In a conventional flash memory, the X-decoder circuit usually provides the required high voltage to the word lines (connected to control gates) of the selected cells. The relatively high bias voltage requires that the X-decoder driver have high device breakdown voltage. This high breakdown voltage not only reduces the speed of reading the cells but also makes the x-decoder circuit difficult to shrink.
The conventional X-decoder puts all its PMOS devices in an N-well and NMOS devices in the same P-substrate of the memory array. When the X-decoder has to provide a positive high voltage, the high voltage is provided from the PMOS and the N-well is applied with a higher or equal voltage to maintain a reverse-biased diode condition of the PMOS device in order to prevent a forward current. The structure, however, is not efficient if a bias condition requires that both positive and negative high voltages be supplied to the word lines of the memory cells in the array.
Another technology called triple-well technology has also been used for the circuit structure of an X-decoder. In a triple-well circuit, the NMOS devices are formed in a P-well, the P-well is embedded in a deep N-well and the deep N-well is formed in a P-substrate. This technology makes it easier to apply a negative high voltage to selected word lines. When providing a negative high voltage, the NMOS devices as well as the embedded P-well are applied with negative high voltage and the deep N-well is grounded or connected to a power supply voltage VDD to maintain a reverse-biased diode condition.
The triple-well technology is not an efficient approach for providing both positive and negative high voltages at the same time either because of the strong constraint in the device breakdown voltage. When both positive and negative high voltages are required, the PMOS and N-well have to be connected to a positive high voltage and the NMOS and P-well have to be connected to a negative high voltage simultaneously. As a result, the absolute difference between the positive and negative high voltages can not exceed the device breakdown voltage to avoid damaging the device.